Jim Pubrick has written an interesting
blog entry detailing how they are using Mono on Second
Life. Second Life is going to use Mono to run the
user-provided scripts with native code (which am told can be
up to 150 times faster than the current engine).

SecondLife is using Mono in a non-conventional way, which I
like to think is one of the benefits of having an open source
engine, they have added a micro-threading implementation.

Microthreading was necessary because some of their
simulations are made up of thousand of threads/routines, and
using the default threads in Mono (which are mapped to
operating system threads) would have been too heavy.

Their micro-threading implementation is different than
Tomi's recent
continuation work in that Second Life has added support to
"migrate" running code across machines.